Bump black from 20.8b1 to 21.9b0

Bumps [black](https://github.com/psf/black) from 20.8b1 to 21.9b0.
- [Release notes](https://github.com/psf/black/releases)
- [Changelog](https://github.com/psf/black/blob/main/CHANGES.md)
- [Commits](https://github.com/psf/black/commits)

---
updated-dependencies:
- dependency-name: black
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <support@github.com>

requirements/testing.txt

@@ -1,6 +1,6 @@
 -r base.txt
 
-black==20.8b1
+black==21.9b0
 mock
 model-mommy
 pytest